Lock autom4te's cache.

Lock autom4te's cache.
* lib/Autom4te/XFile.pm ($me, &name, &lock, &truncate, &seek): New.
* bin/autom4te.in (&Request::save, &Request::load): Use an IO::File
argument instead of a file name, so that the request file remains
open during the whole autom4te run.
($icache_file): New.
(&freeze): Lock the $icache_file.